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UBC Theses and Dissertations

Effects of spatial stock structure and effort dynamics on the performance of alternative assessment procedures for the fisheries of Northern Australia Buckworth, Rik C.


With the world's fisheries in crisis, most fisheries fully- or over-fished, and world catches perhaps exceeding sustainable limits, our capability to monitor and manage fisheries is uncertain. I reviewed these problems, and described ways that spatial complexity compromises monitoring and assessment. Monitoring/ management combinations that are robust to fine-scale dynamics are needed. I developed a closed-loop simulation framework, using the disc equation to distribute fishing effort. A suite of small, spatially-complex fisheries were simulated, and fishery performance was measured, under different monitoring/ management arrangements. Spatial dynamics interacted with monitoring/ control systems, engendering fine scale effects such as biomass erosion, and serial depletion. Performance depended upon control and monitoring information quality. It deteriorated as capacity and hyperstability increased. Poor information / control combinations (CPUE/ TAC) produced poor performance, especially where the stock and effort were concentrated. Effort control with monitoring fishing rates (F) by tagging was risk-averse, performing consistently well across all scenarios. I used a single-stock, age-structured model to assess the Northern Territory Spanish mackerel fishery. This fishery's status is grossly uncertain: available abundance and composition data were uninformative; catch and effort history, biological parameters and stock structure were all uncertain. There was no evidence supporting any increase of the current limit reference point. Genetic mark-recapture might overcome the limitations of conventional tagging. It is suggested for direct F measurement, for routine monitoring. This would entail in situ collection of tissue ("tagging") and subsequent screening of catch samples for matches ("recaptures"), using DNAfingerprinting. I present device designs for in situ tissue collection. Success rates (proportions of strikes yielding tissue) relative to design features were examined. Design and the line on the test vessel on which the tool was deployed interacted strongly. Predicted success rates of the best design are 44-85%, depending upon the line used. Industry participation, entailing daily genetic tagging a set number of fish, would ensure that all members of the fished population have a similar probability of being tagged. In further simulations, genetic tagging outperformed other monitoring methods. Performance improved with small concurrent conventional tagging programs. This methodology could be developed to monitor F in many fisheries.

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